The present invention generally relates to motor servo circuits for magnetic recording and reproducing apparatuses, and more particularly to a motor servo circuit for a magnetic recording and reproducing apparatus, which can carry out a satisfactory servo operation even when a pulse of a vertical synchronizing signal within an input video signal is lacking.
Generally, in a magnetic recording and reproducing apparatus which records and reproduces a video on and from a magnetic tape by use of rotary heads, there is provided a motor servo circuit for controlling the rotation of a motor which rotates the rotary heads. In this type of a motor servo circuit, the rotation of the motor is controlled in the following manner. That is, during a recording mode of the recording and reproducing apparatus, the motor servo circuit compares the phase of a signal which is obtained by detecting the rotation of the motor with the phase of a reference signal which is obtained from a vertical synchronizing signal within an input video signal, and the rotation of the motor is controlled responsive to an error output which is obtained as a result of the phase comparison. On the other hand, during a reproducing mode of the recording and reproducing apparatus, the motor servo circuit uses for the phase comparison a reference signal which is obtained from an output signal of a reference oscillator, instead of using the reference signal which is obtained from the vertical synchronizing signal. Thus, during the recording mode of the recording and reproducing apparatus, a separating circuit for separating the vertical synchronizing signal from the input video signal and a circuit for frequency-dividing the separated vertical synchronizing signal and for producing the reference signal, are used to obtain the reference signal.
However, there are cases where the vertical synchronizing signal cannot be separated from the input video signal in the separating circuit. Such cases are introduced when a D.C. level change occurs in the received video signal at a switching point of a picture, when the video signal receiving condition is poor and ghost images occur, and when the video signal is received under a weak electric field condition. In addition, when the input video signal is obtained from a television camera, for example, and the input video signal is dropped out for some reason, it is also impossible to separate the vertical synchronizing signal in the separating circuit. When the vertical synchronizing signal cannot be separated in the separating circuit and the reference signal is accordingly lacking, the motor servo circuit runs out of synchronism and the motor servo circuit can no longer carry out a correct servo operation to control the rotation of the motor.
For this reason, as will be described later on in the specification in conjunction with a drawing, a conventional motor servo circuit is provided with a detecting circuit for detecting a lack of the vertical synchronizing signal, and a switching circuit responsive to an output detection signal of the detecting circuit, for carrying out a switching and for supplying to a phase comparator the reference signal which is obtained from the output signal of the reference oscillator instead of the reference signal which is obtained from the vertical synchronizing signal. Accordingly, when the vertical synchronizing signal is lacking, the conventional motor servo circuit uses the reference signal which is obtained from the output signal of the reference oscillator. However, in normal practice, the output signal of the reference oscillator and the vertical synchronizing signal within the input video signal, are not in synchronism with each other. Hence, when a switching takes place in the switching circuit and the reference signal supplied to the phase comparator is switched from the reference signal which is obtained from the vertical synchronizing signal to the reference signal which is obtained from the output signal of the reference oscillator, the motor servo circuit does not assume a synchronized state immediately after the reference signal supplied to the phase comparator is switched. In other words, there is a problem in that it takes a certain time for the motor servo circuit to assume the synchronized state and carry out the correct servo operation.
In addition, the detecting circuit described before, is generally designed to detect the lack of the vertical synchronizing signal when several pulses of the vertical synchronizing signal are continuously lacking. For example, the detecting circuit will not detect the lack of the vertical synchronizing signal even when one pulse of the vertical synchronizing signal is lacking. Thus, in a case where on pulse of the vertical synchronizing signal is lacking, the switching circuit described before will not operate. On the other hand, the motor servo circuit is generally designed to operate in synchronism with every other pulse of the vertical synchronizing signal, and no problems will occur even when one of the pulses of the vertical synchronizing signal which do not affect the operation of the motor servo circuit is lacking. However, when one of the pulses of the vertical synchronizing signal which affect the operation of the motor servo circuit is lacking, the motor servo circuit operates so as to synchronize with a subsequent pulse of the vertical synchronizing signal. As a result, an instantaneous phase shift occurs and an instability is introduced in the servo operation. Therefore, there is a problem in that it takes a certain time for the servo operation to return to the normal stable state.